The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches.
The transmission and storage of computer data increasingly relies on the use of codecs (coder-decoders) to compress/decompress digital media files to reduce the file sizes to manageable sizes to optimize transmission bandwidth and memory use. Transform coding is a common type of data compression for data that reduces signal bandwidth through the elimination of certain information in the signal. Sub-band coding is a type of transform coding that breaks a signal into a number of different frequency bands and encodes each one independently as a first step in data compression for audio and video signals. Transform coding is typically lossy in that the output is of lower quality than the original input. Many present compression techniques fail to remedy problems associated with compression artifacts, which are noticeable distortion effects caused by the application of lossy data compression, such as pre-echo, warbling, or ringing in audio signals, or ghost images in video data.
Traditional sub-band audio codecs, such as MP3, use frequency transforms with very good frequency selectivity, such as MDCT (modified discrete cosine transform) operations. These codecs produce very compact representations of tonal signals, but atonal noise can be spread out into many bins, requiring a number of non-zero coefficients to represent this content. For low-bitrate audio coding, the high frequencies are often coded with very few bits, because they are generally perceptually less important than lower frequencies. Since these bands represent a disproportionately large range of frequencies, they cover a large number of transform coefficients, and any non-zero coefficients become very expensive to code in terms of bitrate. Often there are only enough bits for a relatively small number of non-zero coefficients, and the resulting coded signal can sound very tonal, even if the original input signal was not tonal. This can result in the creation of a type of distortion called “birdie” artifacts or musical noise. Birdie artifacts are common in low bitrate MP3 files and typically manifest as metallic tones that appear and disappear at random, and are mainly caused by quantizing the spectrum very coarsely, such that if there are many values in the spectrum that are random, only a few may end up being non-zero after quantization, creating noise that sounds like tones.
Current methods of reducing distortion caused by birdie artifacts include using low-pass filters to reduce the amount of signal to quantize. This approach however does not eliminate these artifacts if the effect is seen in the passband of the filter.
What is needed, therefore, is a method and system that more effectively eliminates birdie artifacts than provided in current audio coding systems.